Nonaqueous electrolyte secondary batteries are widely used as power sources of portable electronic devices. Composite oxides such as LiCoO2 and LiMn2O4 are mainly used as positive electrode active materials of the nonaqueous electrolyte secondary batteries. Moreover, it has been proposed to use a lithium silicate compound Li2MSiO4 (M: a transition metal element) as a positive electrode active material (See Patent Document 1).
In recent years, nonaqueous electrolyte secondary batteries have been considered to be used not only in portable electronic devices but also in vehicles. In order to expand use of the nonaqueous electrolyte secondary batteries as power sources in vehicles, a sufficient amount of lithium resources needs to be secured.
The amount of lithium resources, however, is limited, so battery materials to be used instead of these materials are now under development. Among them, sodium is rich in resources and sodium ion secondary batteries using sodium are being studied.